It will be appreciated by those having ordinary skill in the art that thermal processing of particulate-containing food products is difficult to accomplish in an efficient but effective manner. Particulate-containing food products are also described in the art as multi-phase food products, or as multi-phase foods, in that these products include liquids and solids.
Traditionally, thermal processing of particulate-containing food products involved the placing of the product in individual cans, followed by thermal treatment of the product within the can. The process is generally effective in removing microbial contamination and in providing a food product that is safe for consumption. However, this process is labor and machinery-intensive and time-consuming. Thus, this process lacks efficiency.
Continuous thermal processing generally involves the thermal processing of the food product as a stream or flow in one line while processing the containers or cans in which the food will be stored in another line. The food product is then placed in the container under appropriate conditions wherein microbes and their spores are excluded. Continuous thermal processing thus enables unlimited package size, yielding increased efficiencies and reduced costs to the industry and ultimately to the consumer. Continuous thermal processing is sometimes also called aseptic processing in the art.
However, it is very difficult to uniformly treat all fluid elements of the food product flowing in a continuous process. This difficulty is exacerbated in the case of a multi-phase food product in that there are variations in speed for each solid food particle flowing through the process. Thus, the residence time of each food particle in the flow is a difficult variable to characterize.
As is known in the art, the residence time for a particle in a multi-phase food product traveling through a continuous thermal processing line is that amount of time that the particle resides in a given length or section of the line, or in the entire line itself. Residence time is an important variable because, among other things, it is necessary to the calculation of the lethality per particle for the continuous thermal process. As is also known in the art, lethality can be calculated as a function of time using several equations that require temperature and residence time measurements, among other measurements. Stated differently, lethality is that amount of time a particle is subjected to a temperature sufficient to kill a microbe common to food particles and its spores. An example of such a microbe is Clostridium botulinum.
It is particularly important to capture the residence time measurement for the fastest particle traveling through the process. For the fastest particle, the risk of insufficient lethality is great. Optionally, the food stream can be subjected to excessive heat in the thermal process, but this results in food product that is, in effect, overcooked and therefore not palatable to the consumer. This option is not viable in a commercial setting.
The problems of continuous thermal processing of multi-phase food products are more fully discussed in Tucker, G. S. and Withers, P. M., "Determination of Residence Time Distribution of Food Particles in Viscous Food Carrier Fluids using Hall effect sensors", Technical Memorandum 667, Campden Food and Drink Research Association (CFDRA), Campden, Glos., U.K. (1992) and in "Case Study for Condensed Cream of Potato Soup", Aseptic Processing of Multi-phase Foods Workshop, Nov. 14-15, 1995 and Mar. 12-13, 1996 (published 1997).
Because of the above referenced difficulties, in the United States each continuous thermal process for use in the treatment of food must be described in a document to be filed with the United States Food and Drug Administration (FDA) for approval before it can be implemented in industry. Because of the problems associated with uniform treatment in the continuous thermal process, the FDA subjects these documents, hereinafter referred to as "FDA process filings", "process filings" or "FDA filings", to rigorous scrutiny.
To gain FDA approval, a process filing must demonstrate biovalidation of the process, among other information. As in known in the art, biovalidation refers to data showing that the process was effective in removing contamination of the food product by microbes and their spores. To determine biovalidation, conservative residence time distribution measurements are required. Lengthy test runs must be performed to generate the conservative residence time distribution measurements. Such test runs require a great deal of time and involve the loss of a great deal of the food product, as the food product that is part of the test run cannot be salvaged. The time required for and food product lost in such test runs have prevented the wide scale adoption in the industry of continuous thermal processing of particulate-containing food products.
There have been several attempts to provide methods and systems for characterizing residence time in continuous thermal processing of particulate-containing food products that reduce the time and amount of food product required to validate the process. Currently available methods and systems use the detection of a detectable particle. By the term "detectable particle" it is meant a particle that includes a tracer component that is detectable by sensors used in the method and system and that is attached to or integrated into a carrier component or medium of the particle. The particle is then introduced into the food stream of the continuous thermal process for detection.
Examples of the tracer element include magnetic tracer materials which comprise magnetic particles and metal powders. The particles or powders are mixed in a matrix of solidified plastic polymer or into a an alternative medium such as an alginate gel.
These magnetic tracer materials are only partially compatible with their intended use for measurements of residence time of food particles. Their high density causes an increase in the density of loaded particles which leads to an underestimate of particle velocity. This can lead to an overestimate of hold times and thus yield a calculated process of insufficient lethality to microorganisms and their spores in the food product being processed.
For example, Segner et al., "Biological Evaluation of a Heat Transfer Simulation for Sterilizing Low-Acid Large Particulate Foods for Thermal Packaging", Journal of Food Processing and Preservation, 13:257-274, (1989), reported the use of magnetic implants to tag food particles and measure residence times in thermal systems. Wound copper coils were used as sensor elements, and a single type of magnetic implant was used in the detectable particles. No density compensation for the detectable particles was implemented. The problems with the use of wire coils as magnetic sensors include low sensitivity and low detection reliability at low particle velocities since magnetic flux is detected rather than magnetic field. This approach was followed in "Case Study for Condensed Cream of Potato Soup", Aseptic Processing of Multi-phase Foods Workshop, Nov. 14-15, 1995 and Mar. 12-13, 1996 (published 1997), referenced above.
Tucker, G. S. and Richardson, P. S., "Residence Time Distribution and Flow Behavior of Foods Containing Particles in Thermal Processing", AIChem.E Conference on Food Engineering, Chicago (Feb. 21-24, 1993) (Poster Paper) describes the use of multiple Hall effect sensors mounted around line tubes at different locations to detect the time of passage of magnetically tagged particles through the tube during continuous flow thermal processing. No density compensation of particles was used, and a single particle tag type was applied in detection. Hall effect sensors are more sensitive than wound coil detectors; but, these sensors cannot be used at thermal processing temperatures and have to be positioned a distance away from the tube. This limitation reduces the useable sensitivity. This approach is also described in Tucker, G. S. and Withers, P. M., "Determination of Residence Time Distribution of Food Particles in Viscous Food Carrier Fluids using Hall effect sensors", Technical Memorandum 667, Campden Food and Drink Research Association (CFDRA), Campden, Glos., U.K. (1992), referenced above.
U.S. Pat. No. 5,261,282, issued to Grabowski et al. on Nov. 16, 1993, describes the use of implanted radio frequency transponders to monitor residence times of simulated pasta particles (macaroni) in a continuous thermal system. Multiple transponder i.d. signals were used and density compensation to the carrier fluid density (neutral buoyancy) was applied. The system is limited to large, preferably hollow food particle types due to the size and weight of transponder implants. Additionally, transparent (glass or plastic) tube inserts are necessary to allow for the signal penetration and to enable the detection by external detectors.
Palaniappan et al., "Thermal Process Validated for Foods Containing Particulates", Food Technology, Vol. 51, No. 8, (August 1997) pp. 60-68 used essentially the identical tagging and detection system as described by Segner et al. (1989), but implemented density compensation to the carrier fluid density of the food stream. A single magnetic implant type was used. This work was promising in that it produced the a FDA filing, prepared by Tetra Pak Inc., that received a letter of no rejection from the FDA. This was the first such letter ever issued by the FDA for continuous thermal processing of a multi-phase food product. However, this method, as well as the methods described above, uses a single magnetic tagging implant type and requires that the particle clear the entire observed system or system segment before the next particle is inserted to prevent misidentification. Additionally, all systems using wound coils as sensors are susceptible to occasional non-detection due to low sensitivity and problems at lower velocities.
U.S. Pat. No. 5,021,981 issued to Swartzel et al. on Jun. 4, 1991 and in U.S. Pat. No. 5,159,564 issued to Swartzel et al. on Oct. 27, 1992 each describe method for determining the thermal history of an object, such as a particle of food being treated in a food processing apparatus, and thermal memory cells useful in carrying the methods. The thermal history is determined by detecting changes, after exposure of the object to a thermal treatment, in two thermal calibration materials that have different activation energies and that are placed in or coupled to the object.
Therefore, none of the attempts in the prior art have provided the food processing industry with a method and system that provide a conservative profile of the behavior of food particles in a continuous thermal flow of a particulate-containing food product in real time and in a cost-effective manner; that do not require the use of excessive amounts of food product and time; and that function with a variety of thermal processing systems. Indeed, a suitable detectable particle would have the size and density to provide a conservative residence time measurement as compared to the food particle (i.e., potato, beef cube, etc.) of interest, while containing a sufficient level of magnetic material loading to enable reliable entry and exit detection under realistic processing conditions. By "conservative residence time measurement", it is meant that residence time measurement with the highest likelihood of containing the fastest particle. Such a detectable particle is lacking in the prior art.
What is also needed is a method and system that can conservatively model and calculate residence time in continuous thermal processing of particulate-containing food products. Such a method and system are lacking in the prior art.